According to one report from the National Fire Protection Association data, there are 42,900 fires per year due to electrical equipment. These fires cause $615 Million property damage and 370 lives to be lost every year. Of these fires 15,200 are due to fixed wiring, 7,800 are due to cords and plugs, and 8,400 are due to lamp and light fixtures. The Consumer Product Safety Commission in a 1987 study found that fires are located in every area of residential dwellings; bedrooms, living rooms, kitchens, closet/storage areas, garages, bathrooms, laundry rooms, halls, and dining rooms (in decreasing order of occurrence).
Today circuit breakers only protect overload and short circuit conditions which occur primarily in fixed wiring. The overload protection is provided by the slow heating of a bimetal strip which breaks the circuit and the breaker trips after a specified period of time. The more current that runs through the bimetal, the shorter the time it takes to trip the breaker. Short circuit protection is also provided magnetically, that is, a high level of current trips the breaker instantaneously. The lower limit of the magnetic trip setting is determined by the manufacturer such that the device does not nuisance trip on high inrush loads.
It has been estimated that a large percentage of the fires that occur in residential dwellings can be attributed to "arcing faults." An arc fault is an unintentional electrical discharge characterized by low and erratic current that may ignite combustible materials. Three types of arc faults common to household wiring are parallel, ground, and series. A parallel fault occurs when there is an arc resulting from direct contact of two wires of opposite polarity. A ground fault results when there is an arc between a wire and ground, and a series fault occurs when there is an arc across a break in a single conductor. Arcing can occur and lead to fire in these common situations: frayed appliance extension cords, pierced insulation on electrical construction wire, overheated cords or wires, and appliances where the insulation of the internal wires is impaired. These situations are outside of the normal protection provided by existing circuit breakers.
To address this particular type of problem, industry has developed the Arc Fault Circuit Interrupter (AFCI). AFCI technology was first used to protect the areas surrounding downed utility lines. Advances in electronic technology make arc fault protection available in residential circuits in a cost effective manner. The AFCI adds electronic protection to the "standard" thermal and magnetic protection provided by today's breakers. The arc-fault detection circuitry detects specific arcs that are determined to be likely to cause a fire. The AFCI uses electronics to recognize the current and voltage characteristics of the arcing faults, and interrupts the circuit when the fault occurs.
Another technology developed to address the increasing number of injuries caused from electrical faults is called Ground Fault Circuit Interrupter (GFCI). A GFCI is an inexpensive electrical device that, according to one report, if installed in household branch circuits, could prevent over two-thirds of the approximately 300 electrocutions and thousands of electrical shocks and burns occurring each year in and around the home. The GFCI measures the current flowing through the hot wire and the neutral wire. If the current differs by more than a few milliamps, the presumption is that current is leaking to ground via some other path. This may be because of a short circuit to the chassis of an appliance, or to the ground lead, or through a person. Any of these situations is hazardous, so the GFCI trips, breaking the circuit.
A circuit breaker with both AFCI and GFCI attributes provides a more comprehensive electrical protection solution. For example, GFCIs protect people from injury due to electrical shock using thermal and magnetic protection circuits by tripping the breaker circuit when the leakage to ground exceeds approximately 6 mA. The GFCI can detect phase-to-ground arcs, but cannot detect series arcs or parallel (line-to-neutral) arcs. Since the AFCI can detect both series and parallel arcs, it enhances the protection of the circuit. Operation of an AFCI breaker is described in related U.S. Pat. No. 5,875,087 issued to the assignee of the present application, and incorporated by reference in its entirety herein. AFCI does not replace the function of a GFCI, but it does provide enhanced protection from arcing conditions that may cause a fire. Circuit breakers with both AFCI and GFCI technology require two current sensors. Usually these current sensors are toroidal transformers. The GFCI requires a sensor with very high common mode rejection so only a small differential current produces an output. The toroidal transformer is a good way to achieve this performance.
On the other hand, AFCI circuitry requires a sample of the output current that is proportional to the load current. A toroidal transformer small enough for use in molded-case circuit breakers cannot handle the entire load current without saturating. This is because, after the transformer core saturates, the output characteristics of the transformer are no longer linear in nature. One way to reduce this problem is to use a transformer core with a small air gap. The presence of the air gap greatly reduces the inductance, and consequently the low-frequency cutoff is much higher. However, this adds some cost to the transformer, and since its output characteristics are a derivative of the input current (approximately 6 db/octave in the range of power line frequencies), the resulting signal is more difficult to process.
Circuit breaker testing is another process that must be performed to ensure reliable and safe devices are shipped to the consumer. Production line testing of circuit breakers having both arc fault and ground fault detection circuitry typically requires full-load currents and voltages with complex waveforms to ensure that the desired fault scenarios have been tested. The mere presence of these injurious currents and voltages demands that various precautions with respect to safety and equipment design be enforced to minimize the risk of injury. What is needed is a method for testing AFCI/GFCI circuit breakers during production using low voltages and currents which adequately test the full-load fault conditions of the device.